I think it is too bad that so many people have learned that rule about slipping stitch purlwise unless stated otherwise because it causes a lot of confusion once people start lace knitting.

In the "sl1 k2tog psso" the first stitch should be slipped knitwise not purlwise. Some patterns and books will tell you this (like the Shawls and Scarves: Best of Knitters book) but most assume that you already know it. You can watch a video of how it should be done here (scroll down to sl1, k1, psso):

The thing is, if there's a `rule' that you *always* slip (Xwise), it doesn't necessarily look good in all cases. Some pieces look better if it's slipped purlwise, others knitwise. You just have to decide which goes where